The invention is based on a circuit arrangement and a method for operating an adjusting device driven by an electric motor, which drives at least one adjustable part at which the danger that an object or body part will become caught originates, having an evaluation arrangement that ascertains at least one characteristic variable of the adjusting device as a standard for the adjusting force or adjusting torque and compares it with at least one limit value and if the limit value is exceeded outputs an overload signal. From Published, Non-Examined German Patent Application DE-OS 29 02 683, a control circuit for a window raiser of a motor vehicle is known that in ascertaining an overload signal takes into account the ascertainment of the current flowing through the electric motor of the adjusting device. The motor current is a characteristic variable of the adjusting device and indicates a standard for a force or torque occurring in the adjusting device. A division of the adjusting path into two ranges is provided. In a first range, the detected motor current is utilized to define a limit value, the exceeding of which leads in the second range to tripping of the overload signal. The overload signal on the one hand guards the electric motor against thermal overload and on the other guards against anything becoming caught. The electrical drive power of the electric motor should be defined such that the adjusting force or torque required for the adjusting device can be brought to bear. The catch guard assures that the catching force that acts on an object or on parts of the body of persons is limited to a predetermined value. A further variable in the event of something becoming caught is the spring rate, which indicates the change in force or torque referred to the distance travelled when something gets caught until the overload signal is tripped.
The object of the invention is to provide a circuit arrangement and a method for operating an adjusting device of the type mentioned above, which enable reliable operation and meet a stringent safety demand.